Many hydraulic units, such as hydraulic pumps and motors, are of the variable displacement type. Variable displacement hydraulic units often have a rotating cylinder block with axially movable pistons which engage a tiltable swashplate which varies the stroke of the pistons. The displacement of the hydraulic unit is proportional to the stroke of the pistons within the cylinder block and the tilt angle of the swashplate.
In order to selectively prescribe the position of the swashplate, a displacement control is used to vary the swashplate position in response to a command input. Displacement controls take many forms, but in most cases they allow an operator to manually select a desired swashplate position and the corresponding hydraulic unit displacement.
Many displacement controls include a displacement control valve having an internal spool which is axially movable in response to a command input. The command input shifts the valve spool, as by a differential fluid pressure supplied to pilot pressure chambers formed within a valve body at opposite ends of the spool, to port pressurized fluid to a servo mechanism which, in turn, modulates the swashplate position.
In order to limit the displacement of the spool within the displacement control valve, a spring feedback device interconnects the swashplate with the valve spool. As the valve spool shifts relative to the swashplate, the feedback device gradually develops a force which constrains displacement of the spool. Interconnecting a feedback device between a swashplate and displacement control valve is a particularly troublesome design consideration.
One approach to connecting the swashplate and the control valve has been to position a coil spring coaxially over a portion of the valve spool. The spring is maintained in a compressed, preloaded state between a pair of oppositely spaced spring guides contained in an elongated housing or "spring can", which surrounds the valve spool and is positioned in a specially formed central chamber between the pilot pressure chambers. The spring can is coupled with the swashplate and has opposite ends which are adapted to limit axial separation of the spring guides. When the valve spool is displaced relative to the swashplate, a stop formed on the valve spool engages one spring guide and deflects the feedback spring to generate a feedback force opposing further displacement of the valve spool.
One problem which arises with the known device is that, in order to accommodate the feedback apparatus, as well as a number of fluid metering ports, along the length of the valve, a relatively long valve spool and valve body are required. In addition, the axial bore which guides the spool in the valve body is discontinuous due to the interruption of the central chamber. These requirements for the valve spool and valve body result in undesirable manufacturing and performance characteristics. In addition to requiring significant machining, the long spools tend to bend and bind within the valve body to hinder performance of the valve.
Another problem arises in the assembly of the control valve. When installing a valve spool within a valve body, it is desirable simply to insert the spool through one end of an axial bore extending between the pilot pressure chambers. In order to maintain fluid isolation between the pilot pressure chambers, the bore must be just large enough to receive the valve spool, such that opposite ends of the bore are sealed by the valve spool. However, when it is necessary to fit a large feedback apparatus through the bore end, an increased bore diameter is required and the valve spool is not capable of sealing the pilot pressure chamber.
In order to avoid this problem, known devices have segmented valve spools. One segment supports the feedback apparatus and is inserted through an opening in the sidewall of the valve body and is placed directly into the central chamber. End segments are inserted through the axial bore and are connected with the center segment. Obtaining acceptable manufacturing tolerances and adequately joining the several valve spool segments pose significant problems.
The present invention is directed toward overcoming the problems set forth above in a novel and useful way.